1. Field of the Invention
The invention relates to a syringe comprising a cylinder, a piston rod and a piston disposed for displacement within the cylinder by displacement of the piston rod, whereby when the piston rod is displaced in one direction a medium is drawn into the interior of the cylinder while when the rod is displaced in the opposite direction the medium is expelled from the cylinder.
2. Description of the Prior Art
Such syringes are used, for example, for forcing media through filters. In this case, a filter assembly disposed in a filter holder may be connected to the outlet of the syringe. Such arrangements are used, for example, for the cleaning or sterile filtration of small quantities of liquid. In order, then, to propel the liquid, for example in a quantity of 1 to 50 ml, through the filter assembly which is connected to the syringe, a high pressure is required which has to be generated by hand. In the case of known syringes, the liquid or other medium to be filtered is forced through the filter assembly by an inward movement of the piston rod and thus an inward movement of the piston disposed in the cylinder. Under such circumstances, the hand must apply considerable force. In the case of very dense filter materials or where several filter materials are connected in series, particularly diaphragm filters, filtration in this manner is impossible.
A syringe of this type is disclosed in United States patent application Ser. No. 750,427, filed Dec. 14, 1976.
Where turbid liquids, suspensions or liquids comprising material undergoing precipitation, and which are difficult to separate are employed with a syringe of this type, it may well be possible for the pores of the filter to be completely clogged before the whole liquid to be filtered is expelled from the first pressure chamber. As a result there are two possible dangers for the user of the syringe, that is to say: (1) Owing to the practically unlimited further build-up of pressure in the first pressure chamber on pumping via the second pressure chamber a guage pressure, exceeding the pressure rating of the first cylinder becomes established in the first pressure chamber. The bursting or explosion of the first cylinder and the liquid expelled from it will endanger the operator; (2) even if he notices the clogging of the filter pores promptly and stops pumping, the only possibility for the operator to reduce the gauge pressure obtaining in the first pressure chamber will be that of unscrewing the filter holder from the inlet/outlet duct connector with the inlet/outlet duct pointing upwards. The gauge pressure established in the first pressure chamber will then be released by virtue of the pneumatic working medium emerging through the loosened connection to the filter container. As it flows out of the pneumatic working medium will entrain vestiges of the liquid medium in the inlet/outlet duct and the medium will be expelled from the piston and spool pump. More particularly, in the case of the pressure filtration of toxic and/or corrosive media this will present a substantial source of danger for the user. The problem of unintended and irreversible clogging of the inlet/outlet of the first pressure space can therefore under certain circumstances certainly constitute a substantial source of danger for the user.